Ideally, three-phase AC power, as is used in most industrial environments, employs three conductors of electrical power each providing a pure sine wave of current and voltage, the sine waves having equal and constant amplitude and frequency, and each separated from the others by 120.degree. of phase angle.
With the increased use of the solid-state power electronic devices, having non-linear load characteristics, such as motor controllers having switched triacs which connect and disconnect large loads rapidly, the three-phase power at a given facility may become distorted. Such distortion is characterized by the introduction of harmonics into the fundamental frequency of the AC power. Harmonics cause loss of energy in motors and may effect the efficiency and stability of power supplies for sensitive electronic equipment. For this reason limitations on harmonic levels in AC power have been established in many countries.
The harmonic distortion in an AC power main may be reduced by the use of passive filters incorporating inductors and capacitors that have a series resonance at the harmonic frequencies. These passive filters are placed across the power mains to shunt the damaging harmonics.
Such passive filters have a simple structure but a number of disadvantages: they are bulky and expensive, they do not adapt to changes in harmonic frequencies caused by shifts in the fundamental AC frequency, and they do not account for variations in the series impedance of the power mains.
These disadvantages may be overcome by the use of active filters in which a power amplifier (such as a pulse width modulated (PWM) power supply) is connected directly to the power main to provide a countervailing harmonic current cancelling the distortion. Such active filters may be used with passive filters in a hybrid arrangement in which the PWM power supply is connected to the power mains through a series resonant circuit to block the AC fundamental and hence to decrease loading on the PWM power supply.
In order to cancel the harmonic currents, the currents provided by the active or hybrid filter must be equal and opposite to the undesired harmonic currents. Accordingly, both the phase and amplitude of the harmonic component of the power main must be accurately known. There are a number of methods of extracting harmonic components from a waveform including the use of analog filter circuits and digital signal processing. Analog filters have the disadvantages of being extremely sensitive to the values of their components and thus being subject to drift in filter frequency and degradation in performance. Frequency domain digital signal analysis techniques, such as the Fast Fourier Transform, can be extremely stable but are not presently fast enough to provide accurate real time control necessary for the suppression of harmonics with the current generation of industrial computers.